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Abstract:

A mobile communication system capable of communication control on the
basis of load information transmitted/received between base stations by
using inter-base-station communication, comprises: a base station eNB1; a
base station eNB2 adjacent to the base station eNB1; and at least one
base station eNB3 adjacent to the base station eNB2, wherein after
receiving load information 3 of the base station eNB3 from the base
station eNB3, the base station eNB2 transmits, to the base station eNB1,
load information 2 of the base station eNB2 weighted by using the load
information 3 of the base station eNB3.

Claims:

1. A mobile communication system capable of communication control on the
basis of load information transmitted/received between base stations by
using inter-base-station communication, comprising: a first base station;
a second base station adjacent to the first base station; and at least
one third base station adjacent to the second base station, wherein after
receiving load information of the third base station from the third base
station, the second base station transmits, to the first base station,
load information of the second base station weighted by using the load
information of the third base station.

2. The mobile communication system according to claim 1, wherein, when
transmitting the load information of the second base station weighted
using the load information of the third base station to the first base
station, the second base station transmits, to the first base station,
information indicating that the load information of the second base
station has been weighted.

3. The mobile communication system according to claim 1, wherein the
first base station transmits a transmission request of the load
information of the second base station to the second base station, the
second base station transmits a transmission request of the load
information of the third base station to the third base station in
response to reception of the transmission request from the first base
station, and the third base station transmits the load information of the
third base station to the second base station in response to reception of
the transmission request from the second base station.

4. The mobile communication system according to claim 1, wherein the
first base station transmits a transmission request designating weighting
or no weighting, the second base station transmits, to the first base
station, the load information of the second base station weighted by
using the load information of the third base station, when the
transmission request from the first base station designates weighting,
and transmits, to the first base station, load information of the second
base station not weighted by using the load information of the third base
station, when the transmission request from the first base station
designates no weighting.

5. A mobile communication system capable of communication control on the
basis of load information transmitted/received between base stations,
comprising: a first base station; a second base station adjacent to the
first base station; and at least one third base station adjacent to the
second base station, wherein after receiving load information of the
third base station from the third base station, the second base station
transmits load information of the second base station and the load
information of the third base station to the first base station.

6. The mobile communication system according to claim 5, wherein, after
receiving the load information of the second base station and the load
information of the third base station from the second base station, the
first base station weights the load information of the second base
station using the load information of the third base station.

7. The mobile communication system according to claim 5, wherein, the
first base station transmits a transmission request designating the
presence or absence of the load information of the third base station,
the second base station transmits the load information of the second base
station and the load information of the third base station to the first
base station, when the transmission request from the first base station
designates the presence of the load information of the third base
station, and transmits the load information of the second base station to
the first base station, when the transmission request from the first base
station designates the absence of the load information of the third base
station.

8. A base station that operates as a second base station in a mobile
communication system which includes a first base station, the second base
station adjacent to the first base station, and at least one third base
station adjacent to the second base station, and capable of communication
control on the basis of load information transmitted/received between
base stations, wherein after receiving load information of the third base
station from the third base station, the base station transmits, to the
first base station, load information of the base station weighted by
using the load information of the third base station.

9. A base station that operates as a second base station in a mobile
communication system which includes a first base station, the second base
station adjacent to the first base station, and at least one third base
station adjacent to the second base station, and capable of communication
control on the basis of load information transmitted/received between
base stations, wherein after receiving load information of the third base
station from the third base station, the base station transmits load
information of the base station and the load information of the third
base station to the first base station.

10. A communication method used in a mobile communication system which
includes a first base station, a second base station adjacent to the
first base station, and at least one third base station adjacent to the
second base station and capable of communication control on the basis of
load information transmitted/received between base stations, comprising:
transmitting to the first base station, by the second base station, load
information of the second base station weighted by using load information
of the third base station after receiving the load information of the
third base station from the third base station.

11. A communication method used in a mobile communication system which
includes a first base station, a second base station adjacent to the
first base station, and at least one third base station adjacent to the
second base station and capable of communication control on the basis of
load information transmitted/received between base stations, comprising:
transmitting, by the second base station, load information of the second
base station and load information of the third base station to the first
base station after receiving the load information of the third base
station from the third base station.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a mobile communication system, a
base station, and a communication method, to which an SON technology is
applied.

BACKGROUND ART

[0002] In LTE (Long Term Evolution) standardized in 3GPP (3rd Generation
Partnership Project) which is a group aiming to standardize a mobile
communication system, an SON (Self-Organizing Network) technology, in
which a base station autonomously adjusts a parameter according to a base
station without using any manpower, is applied (refer to Non Patent
Literature 1).

[0003] As one of the SON technology, in order to balance loads between
base stations, there has been proposed technique of adjusting a base
station parameter (specifically, a handover parameter) for determining a
coverage on the basis of load information transmitted/received between
neighboring base stations. Such an optimization technology is called MLB
(Mobility Load Balancing), and is able to improve service quality
provided to a radio terminal. In addition, the coverage indicates the
range of a logical or physical communication area of a base station.

[0004] Specifically, in LTE, the following four types of load information
have been defined (refer to Non Patent Literature 2): (a) the number of
used PRBs (Physical Resource Blocks) each of which is an assignment unit
of a time and frequency resource, (b) a load of a backhaul between a base
station and a core network, (c) a hardware load of a base station, and
(d) a ratio of available communication capacity to a capacity class that
is an index indicating relative communication capacity of a base station.

[0005] In the aforementioned MLB, for example, if a base station detects
that a load of a neighboring base station is lower than that of the self
base station on the basis of load information from the neighboring base
station, the base station adjusts a parameter such that the coverage of
the self base station is reduced with respect to the neighboring base
station. In this way, it is possible to increase the load of the
neighboring base station while reducing the load of the self base
station, resulting in the balancing of the loads.

[0006] Furthermore, for example, if is the base station detects that the
load of the neighboring base station is higher than that of the self base
station on the basis of the load information from the neighboring base
station, the base station adjusts a parameter such that the coverage of
the self base station is expanded with respect to the neighboring base
station. In this way, it is possible to reduce the load of the
neighboring base station while increasing the load of the self base
station, resulting in the balancing of the loads.

[0010] Specifically, even in a case in which a load of a neighboring base
station of a certain base station is low, when the number of radio
terminals in a cell of a base station adjacent to the neighboring base
station (hereinafter, appropriately a "next neighboring base station") is
large, the radio terminals may move from the cell of the next neighboring
base station to a cell of the neighboring base station. Therefore, the
load of the neighboring base station may be increased in the future.

[0011] In such a situation, when the base station reduces the coverage on
the grounds that the load of the neighboring base station is low, it is
not possible to immediately absorb the load when the load of the
neighboring base station increases. As a consequence, there is a problem
that it is not possible to sufficiently improve service quality provided
to a radio terminal.

[0012] Therefore, an object of the present invention is to provide a
mobile communication system, a base station, and a communication method,
by which it is possible to sufficiently improve service quality by
performing communication control in consideration of a load of a next
neighboring base station in addition to a load of a neighboring base
station.

[0013] In order to solve the aforementioned problem, the present invention
has a following characteristic. The characteristic of a mobile
communication system according to the present invention is summarized as
follows. The mobile communication system (mobile communication system 1)
is capable of communication control on the basis of load information
transmitted/received between base stations by using inter-base-station
communication, comprising: a first base station (base station eNB1); a
second base station (base station eNB2) adjacent to the first base
station; and at least one third base station (base station eNB3) adjacent
to the second base station, wherein after receiving load information of
the third base station from the third base station, the second base
station transmits, to the first base station, load information of the
second base station weighted by using the load information of the third
base station.

[0014] According to such a characteristic, after receiving the load
information of the third base station from the third base station, the
second base station transmits, to the first base station, load
information of the self base station (the second base station) weighted
by using the load information.

[0015] In this way, the first base station is able to perform
communication control for the future in consideration of the state of the
load of a next neighboring base station (the third base station), that
is, a potential load of a neighboring base station (the second base
station).

[0016] Consequently, according to the characteristic, MLB is achieved in
consideration of the load of the next neighboring base station in
addition to the neighboring base station, so that it is possible to
provide a mobile communication system capable of sufficiently improving
service quality.

[0017] Another characteristic of the mobile communication system according
to the present invention is summarized as follows. In the mobile
communication system, when transmitting the load information of the
second base station weighted using the load information of the third base
station to the first base station, the second base station transmits, to
the first base station, information indicating that the load information
of the second base station has been weighted.

[0018] Another characteristic of the mobile communication system according
to the present invention is summarized as follows. In the mobile
communication system, the first base station transmits a transmission
request of the load information of the second base station to the second
base station, the second base station transmits a transmission request of
the load information of the third base station to the third base station
in response to reception of the transmission request from the first base
station, and the third base station transmits the load information of the
third base station to the second base station in response to reception of
the transmission request from the second base station.

[0019] Another characteristic of the mobile communication system according
to the present invention is summarized as follows. In the mobile
communication system, the first base station transmits a transmission
request designating weighting or no weighting, the second base station
transmits, to the first base station, the load information of the second
base station weighted by using the load information of the third base
station, when the transmission request from the first base station
designates weighting, and transmits, to the first base station, load
information of the second base station not weighted by using the load
information of the third base station, when the transmission request from
the first base station designates no weighting.

[0020] The characteristic of a mobile communication system according to
the present invention is summarized as follows. The mobile communication
system capable of communication control on the basis of load information
transmitted/received between base stations, comprising: a first base
station; a second base station adjacent to the first base station; and at
least one third base station adjacent to the second base station, wherein
after receiving load information of the third base station from the third
base station, the second base station transmits load information of the
second base station and the load information of the third base station to
the first base station.

[0021] According to such a characteristic, after receiving the load
information of the third base station from the third base station, the
second base station transmits the load information of the second base
station and the load information of the third base station to the first
base station.

[0022] In this way, the first base station can perform communication
control for the future in consideration of a load of a next neighboring
base station (the third base station), that is, a possible load of a
neighboring base station (the second base station).

[0023] Therefore, according to the characteristic above, it is possible to
provide a mobile communication system which sufficiently improves service
quality by achieving MLB in consideration of a load of a next neighboring
base station in addition to a load of a neighboring base station.

[0024] Another characteristic of the mobile communication system according
to the present invention is summarized as follows. In the mobile
communication system, after receiving the load information of the second
base station and the load information of the third base station from the
second base station, the first base station weights the load information
of the second base station using the load information of the third base
station.

[0025] Another characteristic of the mobile communication system according
to the present invention is summarized as follows. In the mobile
communication system, the first base station transmits a transmission
request designating the presence or absence of the load information of
the third base station, the second base station transmits the load
information of the second base station and the load information of the
third base station to the first base station, when the transmission
request from the first base station designates the presence of the load
information of the third base station, and transmits the load information
of the second base station to the first base station, when the
transmission request from the first base station designates the absence
of the load information of the third base station.

[0026] The characteristic of a base station according to the present
invention is summarized as follows. The base station that operates as a
second base station in a mobile communication system which includes a
first base station, the second base station adjacent to the first base
station, and at least one third base station adjacent to the second base
station, and capable of communication control on the basis of load
information transmitted/received between base stations, wherein after
receiving load information of the third base station from the third base
station, the base station transmits, to the first base station, load
information of the base station weighted by using the load information of
the third base station.

[0027] The characteristic of a base station according to the present
invention is summarized as follows. The base station that operates as a
second base station in a mobile communication system which includes a
first base station, the second base station adjacent to the first base
station, and at least one third base station adjacent to the second base
station, and capable of communication control on the basis of load
information transmitted/received between base stations, wherein after
receiving load information of the third base station from the third base
station, the base station transmits load information of the base station
and the load information of the third base station to the first base
station.

[0028] The characteristic of a communication method according to the
present invention is summarized as follows. The communication method used
in a mobile communication system which includes a first base station, a
second base station adjacent to the first base station, and at least one
third base station adjacent to the second base station and capable of
communication control on the basis of load information
transmitted/received between base stations, comprises: transmitting to
the first base station, by the second base station, load information of
the second base station weighted by using load information of the third
base station after receiving the load information of the third base
station from the third base station.

[0029] The characteristic of a communication method according to the
present invention is summarized as follows. The communication method
communication method used in a mobile communication system which includes
a first base station, a second base station adjacent to the first base
station, and at least one third base station adjacent to the second base
station and capable of communication control on the basis of load
information transmitted/received between base stations, comprises:
transmitting, by the second base station, load information of the second
base station and load information of the third base station to the first
base station after receiving the load information of the third base
station from the third base station.

[0030] The present invention can provide a mobile communication system, a
base station, and a communication method, by which it is possible to
sufficiently improve service quality by performing communication control
in consideration of a load of a next neighboring base station in addition
to a load of a neighboring base station.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a network configuration diagram illustrating a network
configuration of a mobile communication system according to the first
embodiment and the second embodiment.

[0032]FIG. 2 is a diagram illustrating an example of a communication
environment of the mobile communication system according to the first
embodiment and the second embodiment.

[0033] FIG. 3 is a block diagram illustrating the configuration of the
base station according to the first embodiment and the second embodiment.

[0034]FIG. 4 is a sequence diagram illustrating the operation pattern of
the mobile communication system according to the first embodiment.

[0035]FIG. 5 is a sequence diagram illustrating the operation pattern 2
of the mobile communication system according to the first embodiment.

[0036]FIG. 6 is a sequence diagram illustrating the operation pattern 1
of the mobile communication system according to the second embodiment.

DESCRIPTION OF THE EMBODIMENT

[0037] With reference to the drawings, the first embodiment, the second
embodiment and other embodiments of the present invention will be
described. In the drawings of each embodiment, the same or similar
reference signs are applied to the same or similar parts.

[0038] In each embodiment as will be described below, the mobile
communication system configured based on LTE (Long Term Evolution)
standard, the specification of which is developed in 3GPP (3rd Generation
Partnership Project). However, the present invention may be applied to
the mobile communication system configured based on another standard.

(1) First Embodiment

[0039] A mobile communication system according to a first embodiment is a
mobile communication system capable of performing communication control
on the basis of load information transmitted/received between base
stations using inter-base-station communication, and includes a first
base station, a second base station adjacent to the first base station,
and at least one third base station adjacent to the second base station.
The second base station receives load information of the third base
station from the third base station, and then transmits load information
of the second base station weighted by using the load information of the
third base station, to the first base station.

[0040] Hereinafter, the first embodiment will be described in the sequence
of (1.1) Overview of mobile communication system, (1.2) Configuration of
base station, (1.3) Operation of mobile communication system, and (1.4)
Effect of the first embodiment.

[0041] (1.1) Overview of Mobile Communications System

[0042]FIG. 1 is a network configuration diagram illustrating a network
configuration of a mobile communication system 1 according to the first
embodiment. The mobile communication system 1 employs an SON (Self
Organizing Network) technology.

[0044] The plurality of base stations eNB constitute E-UTRAN (Evolved-UMTS
Terrestrial Radio Access Network). Each of the plurality of base stations
eNB manages one cell or a plurality of cells serving as a communication
area where a service should be provided to the radio terminal UE.

[0045] The base stations eNB adjacent to one another can communicate
mutually via an X2 interface which is a logical communication channel to
provide inter-base-station communication. Furthermore, each of the
plurality of base stations eNB can communicate with EPC (Evolved Packet
Core), specifically, the MME/S-GW, via an S1 interface.

[0046] The radio terminal UE is a radio communication device carried by a
user, and is also called as "User Equipment". The radio terminal UE is
able to perform switching of a serving base station (or a serving cell),
that is, handover, during communication (referred to as a connected
mode).

[0047] The radio terminal UE performs measurement for a reference signal
received from the base station eNB, and transmits a report (a measurement
report) regarding a measurement result to a serving base station eNB. The
measurement result, for example, includes received power of the reference
signal (RSRP). The measurement report may be transmitted from the radio
terminal UE to the base station eNB, by using an event set by the base
station eNB as a trigger, or may be periodically transmitted from the
radio terminal UE to the base station eNB.

[0048] The base station eNB performs handover control on the basis of the
measurement report received from the radio terminal UE. When the radio
terminal UE receives reference signals from a plurality of base stations
eNB, the measurement report may include a plurality of RSRPs
corresponding to the plurality of base stations eNB. The base station eNB
which received the measurement report determines a base station eNB with
the highest RSRP of the plurality of base stations eNB as a handover
destination, for example, on the basis of the measurement report.

[0049] The mobile communication system 1 supports the aforementioned MLB.
The neighboring base stations eNB adjust a handover parameter as a base
station parameter for determining a coverage on the basis of load
information transmitted/received via the X2 interface. For example, when
a load of one base station eNB is higher than that of a neighboring base
station eNB, the handover parameter is adjusted, so that a coverage of
the one base station eNB is reduced and a coverage of the neighboring
base station eNB is expanded.

[0050] As such a handover parameter, it is possible to use an offset value
for correcting RSRP measured by the radio terminal UE. For example, when
one radio terminal UE is able to receive a radio signal from each of two
base stations eNB, before RSRP1 corresponding to one base station eNB is
compared with RSRP2 corresponding to the other base station eNB, an
offset value for correcting the RSRP1 to be higher is added to the RSRP1.
In this way, it is more likely that the RSRP1 after the offsetting
exceeds the RSRP2. Thus, the one base station eNB is preferentially
selected as a handover destination, and a coverage of the one base
station eNB can be expanded. In addition, in order to avoid unnecessary
handover, a pair of the base stations eNB takes one offset value, and the
offset value is shared by the pair of the base stations eNB.

[0051]FIG. 2 is a diagram illustrating an example of a communication
environment of the mobile communication system 1. Circles in FIG. 2
indicate radio terminals UE performing communication.

[0052] As illustrated in FIG. 2, in the present communication environment,
there are provided a base station eNB1, a base station eNB2 adjacent to
the base station eNB1, and three base stations eNB3a, eNB3b, and eNB3c
adjacent to the base station eNB2. The base station eNB1 corresponds to a
first base station, the base station eNB2 corresponds to a second base
station, and the base stations eNB3a, eNB3b, and eNB3c correspond to
third base stations. Furthermore, in the present communication
environment, the base stations eNB3a, eNB3b, and eNB3c are adjacent to
the base station eNB2, and are not adjacent to the base station eNB2.
That is, the base station eNB2 is a neighboring base station of the base
station eNB1, and the base station eNB3 is a next neighboring base
station of the base station eNB1. Hereinafter, when the base stations
eNB3a, eNB3b, and eNB3c are not particularly distinguished to one
another, the base stations eNB3a, eNB3b, and eNB3c are collectively
referred simply to as the base station eNB3.

[0053] Each of the base station eNB1 and the base station eNB2
transmits/receives load information to/from each other, thereby adjusting
the coverage of the self base station to balance loads. In the present
communication environment, the number of radio terminals UE in a cell of
the base station eNB1 is larger than that of radio terminals UE in a cell
of the base station eNB2, and the load of the base station eNB1 is higher
than that of the base station eNB2. Therefore, the base station eNB1
adjusts a handover parameter to reduce the coverage of the self base
station on the basis of the load information of the base station eNB2.

[0054] However, there are a large number of radio terminals UE in cells of
the base stations eNB3, and these radio terminals UE may move to the cell
of the base station eNB2 and perform handover from the base station eNB3
to the base station eNB2. As a consequence, the load of the base station
eNB2 is increased. However, there is a problem that the base station eNB1
adjusts the handover parameter to reduce the coverage, and therefore, it
is not possible to immediately absorb the load.

[0055] In this regard, loads of the base stations eNB3 are reflected in
the load information transmitted from the base station eNB2 to the base
station eNB1, so that a potential load of the base station eNB2 is
included in the load information, and thus such a problem is solved.
Specifically, the base station eNB2 according to the first embodiment
receives load information of the base stations eNB3 from the base
stations eNB3, and transmits load information of the base station eNB2
weighted by using the load information of the base stations eNB3, to the
base station eNB1.

[0056] (1.2) Configuration of Base Station

[0057] Next, the configuration of each of the base station eNB1 and the
base station eNB2 according to the first embodiment will be described.

[0058] (1.2.1) Configuration of Base Station eNB1

[0059] FIG. 3(a) is a block diagram illustrating the configuration of the
base station eNB1 according to the first embodiment.

[0060] As illustrated in FIG. 3(a), the base station eNB1 includes an
antenna 101, a radio communication unit 110, a network communication unit
120, a storage unit 130, and a control unit 140. The antenna 101 is
connected to the radio communication unit 110. The radio communication
unit 110, the network communication unit 120, and the storage unit 130
are connected to the control unit 140.

[0061] The antenna 101 is used for transmitting and receiving a radio
signal. The antenna 101 may be configured using a plurality of antenna
elements.

[0062] The radio communication unit 110, for example, is configured using
a radio frequency (RF) circuit or a baseband (BB) circuit, and is
configured to perform radio communication through the antenna 101. For
transmission, the radio communication unit 110 performs coding and
modulation of a transmitted signal that is input from the control unit
140, performs up-converting and amplification for the coded and modulated
signal, and then outputs the up-converted and amplified signal to the
antenna 101. For reception, the radio communication unit 110 performs
amplification and down-converting of a received signal that is input from
the antenna 101, performs demodulation and decoding for the amplified and
down-converted signal, and then outputs the demodulated and decoded
signal to the control unit 140.

[0063] The radio communication unit 110 transmits a reference signal.
Furthermore, the radio communication unit 110 receives a measurement
report from the radio terminal UE. Moreover, when it is determined to
perform handover of the radio terminal UE on the base of the measurement
report, the radio communication unit 110 transmits a handover command to
the radio terminal UE.

[0064] The network communication unit 120 is connected to a backhaul
network, and communicates with another network device through the
backhaul network, specifically, the base station eNB2 or the EPC.

[0065] The network communication unit 120 transmits/receives load
information to/from the base station eNB2 using the X2 interface. In LTE,
the following four types of load information have been defined (refer to
Non Patent Literature 2): (a) the number of used PRBs (Physical Resource
Blocks) each of which is an assignment unit of a time and frequency
resource, (b) a load of a backhaul between a base station and a core
network, (c) a hardware load of a base station, and (d) a ratio of
available communication capacity to a capacity class that is an index
indicating relative communication capacity of a base station.

[0066] The storage unit 130, for example, is configured using a memory and
stores various types of information used for control and the like of the
base station eNB1. The control unit 140, for example, is configured using
a CPU, and controls various functions of the base station eNB1.

[0067] The control unit 140 includes a load information processing unit
141 and a handover control unit 142. The function of each of the load
information processing unit 141 and the handover control unit 142, for
example, is performed when CPU serving as the control unit 140 executes a
program stored in the storage unit 130.

[0068] The load information processing unit 141 performs various types of
processing and control related to the load information. Specifically, the
load information processing unit 141 performs the following processing
and control.

[0069] The load information processing unit 141 measures the load of the
base station eNB1 and compares the load of the base station eNB1 with a
threshold value. The load information processing unit 141 performs a
procedure for acquiring the load information of the base station eNB2 in
response to a result of comparing the load of the base station eNB1 with
the threshold value. The procedure includes: a step of controlling, by
the load information processing unit 141, the network communication unit
120 to transmit, to the base station eNB2, a transmission request
(referred to as a Resource Status Request message) for requesting to
start to transmit the load information; a step of receiving, by the
network communication unit 120, a response for the Resource Status
Request message (referred to as a Resource Status Response message) from
the base station eNB2; and a step of receiving, by the network
communication unit 120, a message (referred to as a Resource Status
Update message) including the load information from the base station
eNB2.

[0070] Furthermore, when transmitting the Resource Status Request message
to the base station eNB2, the load information processing unit 141 is
able to transmit a Resource Status Request message which designates
weighting or no weighting. Specifically, the load information processing
unit 141 includes information for requesting weighting in the Resource
Status Request message or information indicating no weighting in the
Resource Status Request message, thereby designating weighting or no
weighting.

[0071] When the network communication unit 120 receives the Resource
Status Update message including the load information of the base station
eNB2, the load information processing unit 141 notifies the handover
control unit 142 of the load information of the base station eNB2.
Furthermore, the load information processing unit 141 notifies the
handover control unit 142 of the load information of the self base
station (the base station eNB1). Hereinafter, the load information of the
base station eNB1 will be referred to as load information 1 and the load
information of the base station eNB2 will be referred to as load
information 2.

[0072] The handover control unit 142 performs various types of processing
and control related to handover. Specifically, the handover control unit
142 performs the following processing and control.

[0073] The handover control unit 142 compares a load indicated by the load
information 1 with a load indicated by the load information 2, and
adjusts a handover parameter to expand the coverage of the self base
station when the load indicated by the load information 1 is lower than
the load indicated by the load information 2. For example, the handover
control unit 142 increases an offset value added to RSRP corresponding to
the self base station, or decreases an offset value added to RSRP
corresponding to the base station eNB2, thereby expanding the coverage of
the self base station with respect to the base station eNB2. Meanwhile,
when the load indicated by the load information 1 is higher than the load
indicated by the load information 2, the handover control unit 142
adjusts the handover parameter to reduce the coverage of the self base
station. For example, the handover control unit 142 decreases the offset
value added to RSRP corresponding to the self base station, or increases
the offset value added to RSRP corresponding to the neighboring base
station, thereby logically reducing the coverage of the self base
station.

[0074] Such a handover parameter is notified by the radio communication
unit 110 to the radio terminal UE and is used when the radio terminal UE
compares RSRPs with one another. Alternatively, the handover parameter
may be used when the handover control unit 142 compares RSRPs with one
another.

[0075] However, in order to adjust a handover parameter, it is necessary
to obtain permission from the base station eNB2. Thus, when a message
(referred to as a Mobility Change Request message) for notifying an
adjusted handover parameter from the network communication unit 120 is
transmitted to the base station eNB2 and the adjusted handover parameter
is confirmed to be permitted, the handover control unit 142 adjusts the
handover parameter.

[0076] (1.2.2) Configuration of Base Station eNB2

[0077] FIG. 3(b) is a block diagram illustrating the configuration of the
base station eNB2 according to the first embodiment.

[0078] As illustrated in FIG. 3(b), the base station eNB2 includes an
antenna 201, a radio communication unit 210, a network communication unit
220, a storage unit 230, and a control unit 240. The antenna 201 is
connected to the radio communication unit 210, and the radio
communication unit 210, the network communication unit 220, and the
storage unit 230 are connected to the control unit 240.

[0079] The antenna 201 is used for transmitting and receiving a radio
signal. The antenna 201 may be configured using a plurality of antenna
elements.

[0080] The radio communication unit 210, for example, is configured using
a radio frequency (RF) circuit or a baseband (BB) circuit, and is
configured to perform radio communication through the antenna 201. For
transmission, the radio communication unit 210 performs coding and
modulation of a transmitted signal that is input from the control unit
240, performs up-converting and amplification for the coded and modulated
signal, and then outputs the up-converted and amplified signal to the
antenna 201. For reception, the radio communication unit 210 performs
amplification and down-converting of a received signal that is input from
the antenna 201, performs demodulation and decoding for the amplified and
down-converted signal, and then outputs the demodulated and decoded
signal to the control unit 240.

[0081] The radio communication unit 210 transmits a reference signal.
Furthermore, the radio communication unit 210 receives a measurement
report from the radio terminal UE. Moreover, when it is determined to
perform handover of the radio terminal UE on the base of the measurement
report, the radio communication unit 210 transmits a handover command to
the radio terminal UE.

[0082] The network communication unit 220 is connected to a backhaul
network, and communicates with another network device through the
backhaul network, specifically, the base station eNB1 or the EPC. The
network communication unit 220 transmits/receives load information
to/from the base station eNB1 using the X2 interface.

[0083] The storage unit 230, for example, is configured using a memory and
stores various types of information used for control and the like of the
base station eNB2. The control unit 240, for example, is configured using
a CPU, and controls various functions of the base station eNB2.

[0084] The control unit 240 includes a load information processing unit
241 and a handover control unit 242. The function of each of the load
information processing unit 241 and the handover control unit 242, for
example, is performed when CPU serving as the control unit 240 executes a
program stored in the storage unit 230.

[0085] The load information processing unit 241 performs various types of
processing and control involved in the load information. Specifically,
the load information processing unit 241 performs the following
processing and control.

[0086] When the network communication unit 220 receives the Resource
Status Request message from the base station eNB1, the load information
processing unit 241 performs a procedure for transmitting the load
information 2 of the self base station (the base station eNB2), to the
base station eNB1. The procedure includes: a step of controlling, by the
load information processing unit 241, the network communication unit 220
to transmit a Resource Status Response message, which is a response for
the Resource Status Request message, to the base station eNB1; a step of
measuring, by the load information processing unit 241, the load of the
self base station and generating the load information 2; and a step of
controlling, by the load information processing unit 241, the network
communication unit 220 to transmit a Resource Status Update message
including the load information 2 to the base station eNB1.

[0087] When the network communication unit 220 receives the Resource
Status Request message, which designates weighting, from the base station
eNB1, the load information processing unit 241 controls the network
communication unit 220 to transmit the Resource Status Request message to
the base stations eNB3. Then, when the network communication unit 220
receives Resource Status Response messages from the base stations eNB3
and then receives Resource Status Update messages from the base stations
eNB3, the load information processing unit 241 weights the load
information 2 of the self base station, by using load information of the
base stations eNB3 (hereinafter, referred to as load information 3),
which is included in the Resource Status Update messages. The weighting,
for example, indicates a process of adding a result, which is obtained by
multiplying the load information 2 of the self base station by a
predetermined coefficient, to a result obtained by multiplying the load
information 3 of the base stations eNB3 by a predetermined coefficient.
Alternatively, the weighting may indicate a process of calculating an
average of the load information 2 of the self base station and the load
information 3 of the base stations eNB3. The load information processing
unit 241 controls the network communication unit 220 to transmit a
Resource Status Update message including the weighted load information 2
to the base station eNB1. When transmitting the load information 2 of the
base station eNB2 weighted by using the load information 3 of the base
stations eNB3, to the base station eNB1, the load information processing
unit 241 includes information indicating that the load information 2 of
the base station eNB2 is weighted, in the Resource Status Update message.

[0088] Since the function of the handover control unit 242 is the same as
that of the handover control unit 142 of the base station eNB1, a
description of the handover control unit 242 will be omitted.

[0089] (1.3) Operation of Mobile Communication System

[0090] Next, an operation pattern 1 and an operation pattern 2 of the
mobile communication system 1 according to the first embodiment will be
described. The operation pattern 1 is an operation pattern of performing
weighting of the load information 2 in the base station eNB2, and the
operation pattern 2 is an operation pattern of not performing weighting
of the load information 2 in the base station eNB2.

[0091] (1.3.1) Operation Pattern 1

[0092]FIG. 4 is a sequence diagram illustrating the operation pattern 1
of the mobile communication system 1 according to the first embodiment.

[0093] In step S101, the base station eNB1 transmits a Resource Status
Request message including information for requesting weighting to the
base station eNB2. The base station eNB2 receives the Resource Status
Request message.

[0094] In step S102, the base station eNB2 transmits a Resource Status
Response message to the base station eNB1 when permitting the Resource
Status Request message from the base station eNB1. The base station eNB1
receives the Resource Status Response message.

[0095] In step S103, the base station eNB2 transmits the Resource Status
Request message to the base stations eNB3 in response to the reception of
the Resource Status Request message including information for requesting
weighting from the base stations eNB1.

[0096] In step S104, the base stations eNB3 transmit a Resource Status
Response message to the base station eNB2 when permitting the Resource
Status Request message from the base station eNB2. The base station eNB2
receives the Resource Status Response message.

[0097] In step S105, the base station eNB2 measures a load of the self
base station (the base station eNB2) according to the Resource Status
Request message from the base station eNB1, and generates load
information 2 indicating a measurement result. In addition, the process
in step S105 may be performed before step S103 or between step S103 and
step S104.

[0098] In step S106, the base stations eNB3 measure loads of their self
base stations (the base stations eNB3) according to the Resource Status
Request message from the base station eNB2, and generate load information
3 indicating a measurement result. Specifically, the base station eNB3a
generates load information 3a, the base station eNB3b generates load
information 3b, and the base station eNB3c generates load information 3c.

[0099] In step S107, the base stations eNB3 transmit a Resource Status
Update message including the load information 3 to the base station eNB2.
The base station eNB2 receives the Resource Status Update message.

[0100] In step S108, the base station eNB2 weights the load information 2,
which was generated in step S105, using the load information 3 received
in step S107. As described above, the weighting indicates a process of
adding a result, which is obtained by multiplying the load information 2
by a predetermined coefficient, to a result obtained by multiplying the
load information 3 by a predetermined coefficient. Alternatively, the
weighting may indicate a process of calculating an average of the load
information 2 and the load information 3.

[0101] In step S109, the base station eNB2 transmits, to the base station
eNB1, a Resource Status Update message including the weighted load
information 2 and information indicating that the load information 2 is
weighted. The base station eNB1 receives the Resource Status Update
message.

[0102] In step S110, the base station eNB1 determines a handover parameter
between the self base station and the base station eNB2, by using the
weighted load information 2 from the base station eNB2. For example, when
the load information 1 indicating that the load of the base station eNB1
is equal to the weighted load information 2, the base station eNB1
considers the handover parameter to be optimized, and maintains the
handover parameter without a change. Meanwhile, when the load information
1 is lower than the weighted load information 2, the base station eNB1
adjusts the handover parameter to expand the coverage of the self base
station. Furthermore, when the load information 1 is higher than the
weighted load information 2, the base station eNB1 adjusts the handover
parameter to reduce the coverage of the self base station with respect to
the base station eNB2. When the base station eNB1 adjusts a handover
parameter of the self base station, the following processes are
performed.

[0103] In step S111, the base station eNB1 transmits a Mobility Change
Request message including the adjusted handover parameter to the base
station eNB2. The base station eNB2 receives the Mobility Change Request
message.

[0104] In step S112, the base station eNB2 transmits a Mobility Change
Acknowledge message to the base station eNB1 when permitting the Mobility
Change Request message. The base station eNB1, the base station eNB2,
receives the Mobility Change Acknowledge message.

[0105] In step S113 and step S114, the base station eNB1 and the base
station eNB2 set the adjusted handover parameter therein.

[0106] In addition, the processes of steps S105 to S109 are repeated until
the base station eNB1 requests the base station eNB2 to stop the
transmission of the load information.

[0107] (1.3.2) Operation Pattern 2

[0108]FIG. 5 is a sequence diagram illustrating the operation pattern 2
of the mobile communication system 1 according to the first embodiment.

[0109] In step S201, the base station eNB1 transmits, to the base station
eNB2, a Resource Status Request message not including information for
requesting weighting or a Resource Status Request message including
information for not requesting weighting. The base station eNB1 receives
the Resource Status Request message.

[0110] In step S202, the base station eNB2 transmits a Resource Status
Response message to the base station eNB1 when permitting the Resource
Status Request message from the base station eNB1. The base station eNB1
receives the Resource Status Response message.

[0111] In step S203, the base station eNB2 measures a load of the self
base station (the base station eNB2) according to the Resource Status
Request message from the base station eNB1, and generates load
information 2 indicating a measurement result.

[0112] In step S204, the base station eNB2 transmits a Resource Status
Update message including the load information 2 to the base station eNB1.
The Resource Status Update message may include information indicating
that the load information 2 is not weighted. The base station eNB1
receives the Resource Status Update message.

[0113] In step S205, the base station eNB1 determines the handover
parameter of the self base station, by using the load information 2 from
the base station eNB2. For example, when the load information 1
indicating that the load of the base station eNB1 is equal to the load
information 2, the base station eNB1 considers the handover parameter to
be optimized, and maintains the handover parameter without a change.
Meanwhile, when the load information 1 is lower than the load information
2, the base station eNB1 adjusts the handover parameter to expand the
coverage of the self base station. Furthermore, when the load information
1 is higher than the load information 2, the base station eNB1 adjusts
the handover parameter to reduce the coverage of the self base station.
When the base station eNB1 adjusts a handover parameter of the self base
station, the following processes are performed. Since the following
processes (steps S206 to s209) are equal to those of the operation
pattern 1, a description thereof will be omitted.

[0114] (1.4) Effect of the First Embodiment

[0115] As described above, after receiving the load information 3 of the
base stations eNB3 from the base stations eNB3, the base station eNB2
transmits load information 2 of the self base station weighted by using
the load information 3, to the base station eNB1. In this way, the base
station eNB1 is able to perform communication control for the future in
consideration of the state of the loads of the base stations eNB3, which
are next neighboring base stations, that is, a potential load of the base
station eNB2 which is a neighboring base station.

[0116] Furthermore, in the first embodiment, when transmitting, to the
base station eNB1, the load information 2 of the base station eNB2
weighted by using the load information 3 of the base stations eNB3, the
base station eNB2 transmits, to the base station eNB1, information
indicating that the load information 2 of the base station eNB2 is
weighted. In this way, the base station eNB1 is able to recognize whether
the load information 2 from the base station eNB2 is weighted, thereby
performing appropriate communication control.

[0117] Moreover, in the first embodiment, in response to the reception of
the Resource Status Request message from the base station eNB1, the base
station eNB2 transmits the Resource Status Request message to the base
stations eNB3. In response to the reception of the Resource Status
Request message from the base station eNB2, the base stations eNB3
transmit the Resource Status Update message including the load
information 3 of the base station eNB3, to the base station eNB2. In this
way, it is possible to acquire the load information 3 only when the load
information 3 is necessary, so that traffic of the backhaul is reduced as
compared with the case in which each base station always
transmits/receives load information.

[0118] In the first embodiment, when the Resource Status Request message
from the base station eNB1 designates weighting, the base station eNB2
transmits the load information 2 of the base station eNB2 weighted by
using the load information 3 of the base stations eNB3, to the base
station eNB1. Meanwhile, when the Resource Status Request message from
the base station eNB1 does not designate weighting, the base station eNB2
transmits the unweighted load information 2 of the base station eNB2, to
the base station eNB1. In this way, the base station eNB1 is able to cope
with both a situation for recognizing the state of the load of the base
station eNB2 including a potential load and a situation for recognizing
the state of the load of the base station eNB2 at the current time point,
thereby performing more appropriate communication control.

(2) Second Embodiment

[0119] In a mobile communication system according to a second embodiment,
after receiving the load information 3 of the base stations eNB3 from the
base stations eNB3, the base station eNB2 transmits the load information
2 of the base station eNB2 and the load information 3 of the base
stations eNB3 to the base station eNB1. Furthermore, after receiving the
load information 2 of the base station eNB2 and the load information 3 of
the base stations eNB3 from the base station eNB2, the base station eNB1
weights the load information 2 of the base station eNB2 using the load
information 3 of the base stations eNB3.

[0120] Hereinafter, the second embodiment will be described in the
sequence of (2.1) Operation of mobile communication system and (2.2)
Effect of the second embodiment. Differences from the first embodiment
will be described and a description identical to that of the first
embodiment will be omitted.

[0121] (2.1) Operation of Mobile Communication System

[0122]FIG. 6 is a sequence diagram illustrating the operation pattern 1
of the mobile communication system 1 according to the second embodiment.
In addition, the operation pattern 2 is equal to that in the first
embodiment.

[0123] In step S301, the base station eNB1 transmits, to the base station
eNB2, a Resource Status Request message including information (next
neighboring load request information) for requesting the load information
3 of the base station eNB3. The base station eNB2 receives the Resource
Status Request message.

[0124] In step S302, the base station eNB2 transmits a Resource Status
Response message to the base station eNB1 when permitting the Resource
Status Request message from the base station eNB1. The base station eNB1
receives the Resource Status Response message.

[0125] In step S303, the base station eNB2 transmits the Resource Status
Request message to the base stations eNB3 in response to the reception of
the Resource Status Request message which designates the presence of the
load information 3 of the base station eNB3, from the base station eNB1.

[0126] In step S304, the base stations eNB3 transmit a Resource Status
Response message to the base station eNB2 when permitting the Resource
Status Request message from the base station eNB2. The base station eNB2
receives the Resource Status Response message.

[0127] In step S305, the base station eNB2 measures a load of the self
base station (the base station eNB2) according to the Resource Status
Request message from the base station eNB1, and generates load
information 2 indicating a measurement result. In addition, the process
in step S305 may be performed before step S303 or between step S303 and
step S304.

[0128] In step S306, the base stations eNB3 measure loads of their self
base stations (the base stations eNB3) according to the Resource Status
Request message from the base station eNB2, and generate load information
3 indicating a measurement result. Specifically, the base station eNB3a
generates load information 3a, the base station eNB3b generates load
information 3b, and the base station eNB3c generates load information 3c.

[0129] In step S307, the base stations eNB3 transmit a Resource Status
Update message including the load information 3 to the base station eNB2.
The base station eNB2 receives the Resource Status Update message.

[0130] In step S308, the base station eNB2 transmits, to the base station
eNB1, a Resource Status Update message including the load information 3
received in step S307 and the load information 2 generated in step S305.
The base station eNB1 receives the Resource Status Update message.

[0131] In step S309, the base station eNB1 weights the load information 2
included in the Resource Status Update message using the load information
3 included in the Resource Status Update message. As described above, the
weighting indicates a process of adding a result, which is obtained by
multiplying the load information 2 by a predetermined coefficient, to a
result obtained by multiplying the load information 3 by a predetermined
coefficient. Alternatively, the weighting may indicate a process of
calculating an average of the load information 2 and the load information
3.

[0132] In step S310, the base station eNB1 determines a handover parameter
between the self base station and the base station eNB2, by using the
weighted load information 2. For example, when the load information 1
indicating that the load of the base station eNB1 is equal to the
weighted load information 2, the base station eNB1 considers the handover
parameter to be optimized, and maintains the handover parameter without a
change. Meanwhile, when the load information 1 is lower than the weighted
load information 2, the base station eNB1 adjusts the handover parameter
to expand the coverage of the self base station. Furthermore, when the
load information 1 is higher than the weighted load information 2, the
base station eNB1 adjusts the handover parameter to reduce the coverage
of the self base station with respect to the base station eNB2. When the
base station eNB1 adjusts a handover parameter of the self base station,
the following processes are performed.

[0133] In step S311, the base station eNB1 transmits a Mobility Change
Request message including the adjusted handover parameter to the base
station eNB2. The base station eNB2 receives the Mobility Change Request
message.

[0134] In step S312, the base station eNB2 transmits a Mobility Change
Acknowledge message to the base station eNB1 when permitting the Mobility
Change Request message. The base station eNB1, the base station eNB2,
receives the Mobility Change Acknowledge message.

[0135] In step S313 and step S314, the base station eNB1 and the base
station eNB2 set the adjusted handover parameter therein.

[0136] In addition, the processes of steps S305 to S309 are repeated until
the base station eNB1 requests the base station eNB2 to stop the
transmission of the load information.

[0137] (2.2) Effect of the Second Embodiment

[0138] As described above, according to the second embodiment, after
receiving the load information 3 of the base stations eNB3 from the base
stations eNB3, the base station eNB2 transmits the Resource Status Update
message including the load information 2 of the base station eNB2 and the
load information 3 of the base stations eNB3, to the base station eNB1.
After receiving the Resource Status Update message including the load
information 2 of the base station eNB2 and the load information 3 of the
base stations eNB3 from the base station eNB2, the base station eNB1
weights the load information 2 of the base station eNB2 using the load
information 3 of the base stations eNB3. In this way, the base station
eNB1 is able to perform communication control for the future in
consideration of the state of the loads of the base stations eNB3, which
are next neighboring base stations, that is, a potential load of the base
station eNB2 which is a neighboring base station.

[0139] Furthermore, in the second embodiment, when the Resource Status
Request message from the base station eNB1 designates the presence of the
load information 3 of the base stations eNB3, the base station eNB2
transmits, to the base station eNB1, the load information 3 of the base
stations eNB3 in addition to the load information 2 of the base station
eNB2. Meanwhile, when the Resource Status Request message from the base
station eNB1 designates the absence of the load information 3 of the base
stations eNB3, the base station eNB2 transmits the load information 2 of
the base station eNB2 to the base station eNB1. In this way, the base
station eNB1 is able to cope with both a situation for recognizing the
state of the load of the base station eNB2 including a potential load and
a situation for recognizing the state of the load of the base station
eNB2 at the current time point, thereby performing more appropriate
communication control.

(3) Other Embodiments

[0140] As described above, the present invention has been described
according to the embodiments. However, it must not be understood that the
discussions and the drawings constituting a part of this disclosure limit
the present invention. From this disclosure, various alternative
embodiments, examples and operational techniques will be apparent to
those skilled in the art.

[0141] In the aforementioned first embodiment, in response to the
reception of the Resource Status Request message from the base station
eNB1, the base station eNB2 transmits the Resource Status Request message
to the base stations eNB3. However, in the case of transmitting the
Resource Status Request message to the base stations eNB3 before
receiving the Resource Status Request message from the base stations
eNB1, it is not necessary for the base station eNB2 to transmit the
Resource Status Request message to the base stations eNB3.

[0142] In the aforementioned each embodiment, the offset value was
described as the handover parameter. However, as well as the offset
value, a handover threshold value to be compared with RSRP may be
adjusted. Furthermore, as well as the adjustment of the handover
parameter, another base station parameter (for example, an antenna tilt
angle or transmission power) may be adjusted, so that a coverage is
physically expanded or reduced. Moreover, communication control based on
load information may include another communication control as well as the
expansion or reduction of the coverage.

[0143] As described above, it must be understood that the present
invention includes various embodiments and the like which are not
described herein.

[0144] Note that the entire content of the Japanese Patent Application No.
2010-257702 (filed on Nov. 18, 2010) is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0145] As described above, a mobile communication system, a base station,
and a communication method according to the present invention are useful
in the radio communication, by which it is possible to sufficiently
improve service quality by performing communication control in
consideration of a load of a next neighboring base station in addition to
a load of a neighboring base station.